If you are unable to access Webmail/Admin when using Kaspersky Antivirus/Internet Security, it may be Kaspersky which is blocking the site.
One user has reported that you can solve this by the following steps:
- Go to the Kaspersky Web Anti-virus Section
- Add the server URL & Webmail port (default 5108) to the
Trusted URLs Section (eg 'http://192.168.0.1:5108)